Dispensers for dispensing individual shots of a liquid are very well known, for example in both private and public bathrooms soap dispensers are commonplace. Similar sorts of dispensers are also known for dispensing industrial detergents and the like, with which a dose of the relevant chemical can be dispensed directly into a washing-up bowl, into a small container such as a cup for transferal to a bucket, or into a bottle slotted into the bottom of the dispenser; in all cases, the intention is that the chemical be dosed in a concentrated form and then diluted with water.
The individual doses of the liquid are drawn from a reservoir (for example a bottle) in the dispenser by means of a cylinder and piston arrangement. When the piston is depressed by the user, the shot is dispensed and the next shot is drawn from the reservoir. A certain amount of time is needed for the subsequent shot to be drawn, this time in part depending on the viscosity of the liquid. Before the subsequent shot is ready, there is the risk that the user could depress the piston again but only receive part of a shot. This means that the correct concentration of liquid would not be obtained, since the same amount of water as for a full shot would tend to be used.
Moreover, in the event that aggressive detergents or sanitizing liquids are being dispensed, there is the problem that users might use excessive concentrations, by putting more than the correct number of shots into the container where the liquid is diluted. In the usual case, of course, only one shot will be used, the size of the shot being adjustable by means of a suitable adjustment to the dispenser. The problem of over-dosing can be reduced if the dispenser is not ready to dispense a further shot for a significant amount of time. It has been attempted to delay the time before which the dispenser can be used again by extending the normal recharge time of the piston arrangement, for example by including a slow release valve in the line between the reservoir of liquid and the discharge cylinder. However, this arrangement still has the disadvantage that a fraction of a shot will be able to be dispensed after the relevant fraction of the delay time has elapsed.